I think because "in" in this case is used to talk about the way something is done. The blood circulates through the body in a certain way, therefore since we would say "in a certain way/ in such a way/in this way", then if we place "the way..." before the description of how something is done, we need to add "in". There are other examples when we use "in" to talk about how an action is done: The houses are all built in the traditional style. We are trying to teach mathematics in a more interesting way.